Noble metal sulphides are widely known in the field of electrocatalysis; in particular, electrocatalysts based on rhodium and ruthenium sulphide are currently incorporated in gas-diffusion electrode structures for use as oxygen-reducing cathodes in highly aggressive environments, such as in the depolarised electrolysis of hydrochloric acid.
Noble metal sulphide electrocatalysts of the prior art are for instance prepared by sparging hydrogen sulphide in an aqueous solution of a corresponding noble metal precursor, usually a chloride, for instance as disclosed in U.S. Pat. No. 6,149,782, entirely incorporated herein as reference, which is relative to a rhodium sulphide catalyst. The synthesis of noble metal sulphide catalysts with hydrogen sulphide in aqueous solutions is conveniently carried out in the presence of a conductive carrier, in most of the cases consisting of carbon particles. In this way, the noble metal sulphide is selectively precipitated on the carbon particle surface, and the resulting product is a carbon-supported catalyst, which is particularly suitable for being incorporated in gas-diffusion electrode structures characterised by high efficiency at reduced noble metal loadings. High surface carbon blacks, such as Vulcan XC-72 from Cabot Corp./USA are particularly fit to the scope.
A different procedure for the preparation of carbon-supported noble metal sulphide catalysts consists of an incipient wetness impregnation of the carbon carrier with a solution of a noble metal precursor salt, for instance a noble metal chloride, followed by solvent evaporation and gas-phase reaction under diluted hydrogen sulphide at ambient or higher temperature, whereby the sulphide is formed in a stable phase. This is for instance disclosed in US 2004/0242412, relating to a ruthenium sulphide catalyst.
A more advanced manufacturing process for noble metal sulphide catalysts is further disclosed in U.S. Pat. No. 6,967,185, entirely incorporated herein as reference, and consists of reacting a noble metal precursor with a thio-compound in an aqueous solution free of sulphide ions; in this way, a catalyst substantially equivalent to the one of U.S. Pat. No. 6,149,782 is obtained avoiding the use of a highly hazardous and noxious reactant such as hydrogen sulphide.
Although the catalysts disclosed in the above referenced documents proved of utmost importance for the successful commercialisation of hydrochloric acid electrolysers, they still presents some limitations in terms of activity and of stability to the particularly aggressive environment typical of such application and consisting of a hydrochloric acid solution containing significant amounts of dissolved chlorine and oxygen.
As regards the activity, noble metal sulphides precipitated by the methods of the prior art are all prepared by discrete reduction stages producing a mixture of different crystalline phases with different valences and stoichiometry, some of which present a poor electrochemical activity or none at all. Moreover, some of the most active formulation consist of ternary compounds which cannot be reliably prepared by the environmentally friend method of U.S. Pat. No. 6,967,185; the only viable process for obtaining ternary compounds, such as RuxCozSy which is also very attractive in terms of cost, is the one disclosed in US 2004/0242412, still relying on hydrogen sulphide as reactant species.
As concerns the stability, the above mentioned mixed-valence systems comprised of different crystalline phases typical of the catalysts of the prior art inevitably results to some extent in the formation of less stable phases such as zerovalent metals, metal oxides and non-stoichiometric perovskites. Although rhodium and ruthenium sulphides are much more stable than any other electrocatalyst for oxygen reduction of the prior art in the hydrochloric acid electrolysis environment, some leakage of noble metal is still detectable, especially when the cell is shut-down for maintenance.